The Mechanism of Adaptation 3 4 l 



and grows in size. If much blood is lost in hemorrhage the 

 activity of the blood-forming organs is increased, and they 

 send more corpuscles and plasma into the circulation; for 

 there is a certain equilibrium between the activity of the 

 blood-forming organs and the quantity of blood in circula- 

 tion. If white corpuscles are destroyed by x-rays the lymph- 

 oid tissues are stimulated to send an excess of leucocytes 

 into the blood, to compensate this deficiency. 



This tendency to equilibrium is probably one of the most 

 important physiological processes in the regulations and 

 adaptations of organisms. A similar tendency is found in 

 the inorganic world; when the osmotic pressure between 

 two fluids separated by a permeable membrane is unequal, 

 equilibrium is automatically restored; when the gas tension 

 differs on two sides of a permeable membrane, diffusion oc- 

 curs through the membrane until the tension is equal on both 

 sides; when the oxygen or carbon-dioxide tension in the 

 blood differs from that in the tissues or in the lungs, there 

 is an exchange of gases until equilibrium is reached; a chemi- 

 cal reaction proceeds in the direction of equilibrium, and if 

 an excess of products is formed in one direction the reaction 

 may sometimes reverse and go in the other direction until 

 equilibrium is restored. Such cases seem to be analogous 

 with the compensatory regulations of organisms, but the 

 balance between one physiological process and another or 

 between the organism and the environment is not only vastly 

 more complex than these inorganic equilibria, it is self-pre- 

 servative and useful; and it is this quality of usefulness or 

 fitness for which we are seeking an explanation. 



It is possible that some of these individual adaptations 

 belong to the fundamental and original properties of living 

 things and as such are not to be explained by any theory of 

 evolution ; for it must not be forgotten that organic evolu- 



